Understand that Google is continuously rewriting their infrastructure to handle increased scale. That's what they have 25k employees for.

Jeff Dean's rule of thumb is that you should build a growth factor of 10 into the design, but any more than that and you will probably have to re-architect anyway. So going from 10,000 to 1 million and 1 million to 100 million are probably roughly equivalent in difficulty.

The big difference between blog hosting and things like Twitter (or Reddit, or Digg, etc) is that blogs are independent, so adding servers scales you up linearly. When you are looking at X's blog, basically everything you see is coming from one server.

You will have to have something that deals with mapping URLs in the unified logical name space of your site to the individual servers that the particular blogs on--that's the part that you can't just throw machines at and get good results.

With the social sites, you can't isolate things easily, because what a given person sees at any time is drawn from an ever changing set of content from other users, with each viewer drawing from a different set.

Based on the stats on the site, I calculate that Wordpress.com is averaging about 10 writes and around 8800 reads per second.

edit: Correction, ~880 reads per second.